This invention relates to a method of manufacturing rolled titanium alloy sheets, and more specifically to a method of manufacturing rolled titanium alloy sheets with excellent strength and ductility, having a uniform, equiaxed .alpha. crystal structure free from anisotropy and prevented from undergoing surface cracking during the process of hot rolling.
Titanium alloys, which combine high specific strength with outstanding corrosion resistance, have enjoyed a steady increase in usage in the aircraft and space industries and also in ground fields for applications in various installations. The widespread usage has brought with it the development of many different titanium alloys, including Ti-Al-V, Ti-Al-Sn, Ti-Mn, Ti-Al-Mn, Ti-Al-Mo-V systems, etc..
Titanium alloys form a group of materials difficult to work, and the literature on the manufacture of their worked products has been rather scanty. Generally, however, it is believed that an equiaxed .alpha. crystal structure excellent in mechanical properties can be obtained by working the alloys through forging or rolling with the highest possible degree of working done in the .alpha.+.beta. region. In connection with forgings, it has been reported that combining forging operation in excess of a given rate of working with heat treatment at a .beta.-region temperature renders it possible to refine and uniformalize the grain size of .alpha. grain (Japanese Patent Application Publication No. 8099/1981). As regards rolled products, it has been proposed to produce an isotropic, fine-grained crystal structure by coupling at least total draft 70% by hot rolling with a treatment for forming an equiaxed .alpha. crystal structure wherein cooling and reheating are carried out under the specified conditions (Japanese Patent Application Public Disclosure No. 25423/1983).
However, those methods of the prior art inevitably leave some partial .alpha. phase behind that is not of an equiaxed .alpha. crystal structure, thus presenting a reliability problem of the products. In the case of forgings, there are marked scatters of structure longitudinally of the forging direction and in the cross section. Even with rolled products it is known that, because the .alpha. phase of titanium alloys represents a hexagonal close-packed crystal structure, substantial mechanical anisotropy develops in the alloys with the directions of rolling and at right angles to the rolling direction. Titanium alloy products, designed for use in severe service environments such as high temperatures, strong corrosive attacks, and heavy loads, are required to exhibit high reliability. Since rolling is basically advantageous over forging in quality of products and in operation efficiency, it is essential to establish a titanium alloy rolling method which will largely control or eliminate the presence of residual .alpha. phase that does not form the equiaxed crystal structure, without inducing mechanical anisotropy, in order to meet the growing requirements there for in various fields.